EMT 251
Introduction to IC Design
(Pengantar Rekabentuk Litar Terkamir)
Semester II 2011/2012
MOS theory
Wednesday, May 9, 12
Lecture 11: MOS transistor theory
• Last time
– Gate layouts and stick diagrams
• This time
– MOS transistor theory (ideal case)
Wednesday, May 9, 12
The Big Picture
So far, we have treated transistors as ideal
switches
An ON transistor passes a finite amount of current
Depends on terminal voltages
Derive current-voltage (I-V) relationships
Transistor gate, source, drain all have capacitance
I = C (ΔV/Δt) --> Δt = (C/I) ΔV
Capacitance and current determine speed
Wednesday, May 9, 12
MOS Transistor symbol
Wednesday, May 9, 12
gate-oxide-body sandwich =
capacitor
Operating modes
• Accumulation
• Depletion
• Inversion
• The charge
accumulated is
proportional to the
excess gate-channel
voltage (Vgc-Vt)
Wednesday, May 9, 12
Terminal Voltages
Mode of operation depends on Vg, Vd, Vs
Vgs = Vg – Vs
Vgd = Vg – Vd
Vds = Vd – Vs = Vgs - Vgd
Source and drain are symmetric diffusion terminals
By convention, source is terminal at lower voltage
Hence Vds ≥ 0
nMOS body is grounded. First assume source is 0 too.
Three regions of operation
Cutoff
Linear
Saturation
Wednesday, May 9, 12
nMOS Cutoff
No channel
Ids = 0
Wednesday, May 9, 12
nMOS Linear
Channel forms
Current flows from d to s
e- from s to d
Ids increases with Vds
Similar to linear resistor
Wednesday, May 9, 12
nMOS Saturation
Channel pinches off
Ids independent of Vds
We say current saturates
Similar to current source
Wednesday, May 9, 12
The MOS transistor has three regions of
operation
Wednesday, May 9, 12
The MOS transistor has three regions of
operation
NMOS transistor, 0.25um, Ld = 10um, W/L = 1.5, VDD = 2.5V, VT = 0.4V
Wednesday, May 9, 12
How to calculate the current value?
Wednesday, May 9, 12
How to calculate the current value?
• MOS structure looks like parallel plate capacitor while
operating in inversion
– Gate – oxide – channel
Wednesday, May 9, 12
How to calculate the current value?
• MOS structure looks like parallel plate capacitor while
operating in inversion
– Gate – oxide – channel
• Qchannel = CV
Wednesday, May 9, 12
How to calculate the current value?
• MOS structure looks like parallel plate capacitor while
operating in inversion
– Gate – oxide – channel
• Qchannel = CV
• C = εoxWL/tox = CoxWL (where Cox=εox/tox)
Wednesday, May 9, 12
How to calculate the current value?
• MOS structure looks like parallel plate capacitor while
operating in inversion
– Gate – oxide – channel
• Qchannel = CV
• C = εoxWL/tox = CoxWL (where Cox=εox/tox)
• V = Vgc – Vt = (Vgs – Vds/2) – Vt
Wednesday, May 9, 12
Carrier velocity is a factor in determining the
current
• Charge is carried by electrons
• Carrier velocity v proportional to lateral E-field
bet ween source and drain
• v = μE
μ called mobility
• E = Vds/L
• Time for carrier to cross channel:
t=L/v
Wednesday, May 9, 12
I=Q/t
• Now we know
– How much charge Qchannel is in the channel
– How much time t each carrier takes to cross
Wednesday, May 9, 12
In linear mode (Vgs > Vt & Vds < Vgs-Vt)
Wednesday, May 9, 12
In linear mode (Vgs > Vt & Vds < Vgs-Vt)
Can be ignored for small
Vds
Wednesday, May 9, 12
In linear mode (Vgs > Vt & Vds < Vgs-Vt)
Can be ignored for small
Vds
ØFor a given Vgs, Ids is proportional (linear) to Vds
Wednesday, May 9, 12
In saturation mode (Vgs > Vt and Vds ≥ Vgs-Vt)
Wednesday, May 9, 12
In saturation mode (Vgs > Vt and Vds ≥ Vgs-Vt)
ØNow drain voltage no longer increases current
Wednesday, May 9, 12
Operation modes summary
–
–
–
–
–
Wednesday, May 9, 12
0.6 micron process
tox = 100 Å
µ = 350 cm2/V*s
Vt = 0.7 V
W/L = 4/2 λ
PMOS is similar
Wednesday, May 9, 12
What happens when we construct a INV (PMOS
+NMOS)?
Wednesday, May 9, 12
Inverter voltage transfer function
A
B
C
D
Wednesday, May 9, 12
E
Summary :Inverter
voltage transfer function
Wednesday, May 9, 12
Summary :Inverter
voltage transfer function
Wednesday, May 9, 12
Inverter current transfer function
Wednesday, May 9, 12
Inverter current transfer function
Wednesday, May 9, 12
Inverter current transfer function
Wednesday, May 9, 12